Field of the Invention
The present invention relates to an automated robotic assembly system configured to automatically assemble a device by fastening various components constituting the device by means of an assembly robot.
Description of the Related Art
In assembling devices, unmanned, automated assembly systems are used in various fields such that various components are automatically fastened by assembly robots to assemble the devices.
For example, an automated assembly system has been developed in which work to mount various components on a printed circuit board used in an electronic device is also unmanned and assembly work is performed by an assembly robot. If stress above a certain level is applied to the printed circuit board during the assembly work, however, the printed circuit board may possibly be distorted so that soldered areas of component-mounting portions are cracked or semiconductor components such as capacitors are broken. Since the operation of the electronic device is not affected by cracks in the capacitors immediately after their occurrence, moreover, defects, if any, of the device cannot be detected at the time of product shipment inspection but may be revealed after shipment to the field, in some cases. In view of reliability, therefore, there is a method of preventing the electronic device from being stressed as it is assembled.
Japanese Patent Application Laid-Open No. 2002-134853 discloses a method in which a strain gage is provided in advance on the substrate of a printed circuit board so that distortion of the printed circuit board can be measured by measuring the resistance of the resistor of the strain gage. According to this method, the printed circuit board is rejected as a non-conforming product if the resulting measured value (distortion) exceeds a predetermined value. Also, this patent document discloses a method in which a electrically-conducting path of a stress detecting pattern is provided in advance as a stress detecting member around a component mounting area on the surface of the printed circuit board so that the resistance of the electrically-conducting path of the stress detecting pattern can be measured before a surface mounted component is attached to the printed circuit board. Also, according to this method, the resistance of the electrically-conducting path of the stress detecting pattern is measured again after the surface mounted component is attached to the printed circuit board. Consequently, if the difference between measured values obtained before and after the attachment of the surface mounted component is not less than a predetermined value, the printed circuit board is considered to be stressed and is rejected as a non-conforming product. Thus, if the printed circuit board is stressed, a part of the electrically-conducting path of the stress detecting pattern is disconnected or deformed. Once the electrically-conducting path is disconnected or deformed, it can never be restored as before. Based on the change in the resistances of the electrically-conducting path before and after the mounting of the components on the printed circuit board, therefore, such a history is determined that the printed circuit board is subjected to stress above a certain level as the components are mounted and assembled, so that defects, if any, of the printed circuit board mounted with the components can be detected.
According to the conventional technique disclosed in Japanese Patent Application Laid-Open No. 2002-134853, it is detected whether or not the printed circuit board mounted on which the components is mounted is distorted. This conventional technique determines whether or not the stress above the certain level is applied to the printed circuit board, based on the change in the resistances of the electrically-conducting path of the stress detecting pattern before and after the mounting of the components on the printed circuit board. As the surface mounted components are attached to this printed circuit board, the printed circuit board is highly stressed so that it is strongly bent and undergoes deformation (so-called plastic deformation) of such a level that it cannot be restored to its normal shape. The printed circuit board with the resistance of the electrically-conducting path of the stress detecting pattern changed is detected by detecting the deformation. Thus, defects, if any, of the printed circuit board that is not elastically deformed despite the high stress thereon cannot be detected.
However, many of printed circuit boards generally have elasticity and their deformation caused on an actual manufacturing floor falls within an elastic region that allows to restore to their original shape. Even elastic deformation may possibly affect (e.g., break) components on the printed circuit boards. According to the conventional technique described in Japanese Patent Application Laid-Open No. 2002-134853, the distortion and the resistance of the electrically-conducting path of the stress detecting pattern after plastic deformation of the printed circuit board are measured. Therefore, the plastic deformation of the printed circuit board cannot be prevented and distortion or elastic deformation of the printed circuit board, which may break the components or reduce the quality of soldering, cannot be detected.
Also in some automated assembly systems for devices other than the above-described electronic device with the mounting components attached to the printed circuit board, distorted or stressed assembly components may be assembled in their automated assembly operation. In this case, failures, if any, cannot be detected at the time of the assembly but may be actualized during prolonged use of the assembled device.